1. Field of the Invention
The present invention relates to sensors that detect the presence or concentration of a particular substance using a radiant source, a photodetector and indicator molecules, which have an optical characteristic that is affected by the presence of the substance. Such sensors are referred to herein as “optical sensors.” In one aspect, the present invention relates to systems and methods for extending the useful life of optical sensors.
2. Discussion of the Background
U.S. Pat. No. 5,517,313, the disclosure of which is incorporated herein by reference, describes a sensing device comprising a radiant source (e.g., a light-emitting diode—“LED”), fluorescent indicator molecules, and a photoelectric transducer (e.g., a photodiode, phototransistor, photomultiplier, or other photodetector). The sensing device may also include a high-pass or bandpass filter. Broadly speaking, in the context of the field of the present invention, an indicator molecule is a molecule having one or more optical characteristics that are affected by the local presence of a particular substance. U.S. Pat. No. 6,330,464, the disclosure of which is incorporated herein by reference, also describes an optical-based sensing device.
In the device according to U.S. Pat. No. 5,517,313, the radiant source (a.k.a., the “light” source) is positioned such that radiation (e.g., visible light or other wavelengths of electromagnetic waves) emitted by the radiant source strikes the fluorescent indicator molecules, thereby causing the indicator molecules to fluoresce. The high-pass filter is configured to allow the radiation emitted by the indicator molecules to reach the photoelectric transducer while filtering out scattered radiation from the light source.
The fluorescence of the indicator molecules employed in the device is modulated (i.e., attenuated or enhanced) by the local presence of a particular substance. For example, the orange-red fluorescence of the complex tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) perchlorate is attenuated by the local presence of oxygen. Therefore, this complex can be used as the indicator molecule in an oxygen sensor. Indicator molecules whose fluorescence properties are affected by various other substances are known as well. Furthermore, indicator molecules which absorb light, with the level of absorption being affected by the presence or concentration of a particular substance, are also known. For example, U.S. Pat. No. 5,512,246, the disclosure of which is incorporated by reference, discloses compositions whose spectral responses are attenuated by the local presence of polyhydroxyl compounds such as sugars.
Advantageously, the photoelectric transducer element of the device is configured to output a signal that is a known function of the amount of light incident thereon. Thus, because the high-pass filter allows only the light from the indicator molecules to reach the photosensitive element, the photoelectric transducer outputs a signal that is a function of the amount of light coming from the indicator molecules. And because the amount of light coming from the indicator molecules is a function of the concentration of the local substance, the signal outputted by the photoelectric transducer can be calibrated to be indicative of the concentration of the local substance. In this manner, one can detect the presence or concentration of a particular substance.
One particular challenge in commercializing such optical sensors as the one described above is to provide for a useful period of shelf and/or operational lifetime. The standard electronic components commonly used in such sensors have useful lifetimes typically exceeding 10 years or more, which is adequate for most commercial products. However, the chemical components of these hybrid sensors (e.g., the indicator molecules) must also support extended lifetime product stability in order to meet the practical criteria of commercial utility.
Unfortunately, light catalyzed oxidation (also termed photo-oxidation or photobleaching) is a common photochemical reaction that occurs with many indicator molecules. In this reaction, when an indicator molecule is excited by a particular incident wavelength of electromagnetic energy, an electron is elevated to an excited energy state. While in the excited state, the molecule can (and does) undergo a reaction with ambient oxygen that results in an irreversible addition of oxygen to the molecular structure of the molecule. The oxidized product species is typically no longer fluorescent, and therefore no longer useful.
When the molecule is in this “non-functioning” state, the molecule is said to be photobleached. A typical half life for this photobleaching reaction is on the order of hours. An example of an indicator molecule that becomes photobleached within hours is anthracene (the photo-oxidized product, anthraquinone, is not fluorescent).
A sensor that utilizes fluorescent indicator molecules as a component to recognize and convert the presence of a substance into a measurable signal is limited in its useful life by the degradation and ultimate loss of signal caused by photobleaching (or photo-oxidation). The microelectronic components of an optical sensor may have useful lifetimes exceeding 10 years, whereas the half-life of the important indicator chemistry component may last only hours or days. This incompatibility in component operational lifetime ultimately limits a product to the shorter useful life of the indicator chemistry.
Therefore, a need exists to compensate for these extreme mismatches in component lifetime to permit commercialization of such products.